1. Field of the Invention
The present invention relates to a switch arrangement, and more particularly to an inductive proximity switch including an oscillator and a sensor resonant circuit which exhibits the same switching distance for both electrically conductive nonmagnetic nonferrous metal (NF) objects and ferromagnetic (FE) objects.
2. Description of Related Art
When an electrically conductive object approaches a coil, the effective inductance of the coil is reduced due to a mutual inductance between the coil and the object caused by electrical currents which are magnetically induced in the object. Furthermore, the quality Q of the coil is reduced due to ohmic losses in the object caused by the induced currents. These effects can be exploited in a proximity switch by detecting various events indicating the approach of an electrically conductive object. For example, it is possible to detect a decrease in the amplitude of an oscillator output, or a cessation of oscillation of an oscillator, due to damping of a sensor coil caused by the approach of an electrically conductive object. While it is possible to detect a change in *the frequency of an oscillator due to a decrease in the inductance of a sensor coil caused by the approach of an electrically conductive object, it is rare to do so.
When a coil is approached by a ferromagnetic object, i.e. an object having a very high magnetic permeability, in addition to the above-described effects resulting from the electrical conductivity of the object, the inductance of the coil is increased due to an increase in the reluctance of the coil caused by the ferromagnetic properties of the object. These effects can be exploited in a proximity switch in a manner similar to that described above.
The difference between the effects on a coil due to the approach of an electrically conductive nonmagnetic object, especially an object made of a nonferrous metal such as copper or aluminum, and the approach of an object made of a ferromagnetic metal, especially an object made of a ferrous metal, can be inconsequential in certain instances, such as when the proximity switch is only to be used to detect the approach of objects made of the same material. However, the difference between the effects can be useful in other instances, such as when the proximity switch is to be used to differentiate between approaching objects made of different materials, rather than merely to detect the approach of objects made of the same material. Finally, there are also instances where the difference between the effects can be disadvantageous, such as when the proximity switch is to be used to detect the approach of objects to a specific distance regardless of the material of which they are made, as is the case in the present invention.
An inductive proximity switch which exhibits the same switching distance for both electrically conductive nonmagnetic nonferrous metal (NF) objects and ferromagnetic metal (FE) objects has been proposed in West German Patentschrift DE 37 14 433 C1, corresponding to U.S. Pat. No. 4,879,531. This switch includes an LC oscillator having a frequency-determining resonant circuit and a sensor resonant circuit. The sensor resonant circuit exhibits different impedance-frequency characteristic curves for the approach of NF and FE objects to a predetermined switching distance. The curves for the NF and FE objects intersect at a critical response point corresponding to a critical frequency f.sub.0 and a critical impedance Z.sub.0. The resonant frequency of the frequency-determining resonant circuit is set to f.sub.0 and the impedance of the sensor resonant circuit at f.sub.0 is set to Z.sub.0. The switch is triggered when the impedance of the sensor resonant circuit becomes equal to Z.sub.0 due to the approach of either an NF object or an FE object to the predetermined switching distance. However, the LC oscillator used in this proposed switch is subject to marked frequency fluctuations which can result in different switching distances for NF and FE objects. As will be explained below, this problem is solved in the present invention by the use of an oscillator which is not subject to frequency fluctuations, such as an oscillator controlled by a ceramic or quartz resonator.
West German Patentschrift 32 21 888 and West German Offenlegungsschrift 33 20 509 propose proximity switches which exhibit different switching distances for NF and FE objects and which are made less sensitive to external influences such as moisture and temperature and to changes in the characteristics of switch components by regulating an oscillator frequency in accordance with the resonant frequency of a sensor resonant circuit. However, this technique cannot be used in the proximity switch of the present invention which exhibits the same switching distance for both NF and FE objects because, as will be explained below, a constant oscillator frequency must be maintained to achieve the same switching distance for both NF and FE objects. As will be explained below, the use of an oscillator controlled by a ceramic or quartz resonator makes the proximity switch of the present invention less sensitive to external influences.